Recent debris flow occurrences associated with glaciers in the Alps

Abstract

Debris flows from glacier forefields, triggered by heavy rain or glacial outbursts, or damming of streams by ice avalanches, pose hazards in Alpine valleys (e.g. the south side of Mount Blanc). Glacier‐related debris flows are, in part, a consequence of general glacier retreat and the corresponding exposure of large quantities of unconsolidated, unvegetated, and sometimes ice‐cored glacial sediments. This paper documents glacier‐related debris flows at 17 sites in the Italian, French, and Swiss Alps, with a focus on the Italian northwest sector. For each case data are provided which describe the glacier and the instability. Three types of events have been recognized, based on antecedent meteorological conditions. Type 1 (9 documented debris flows) is triggered by intense and prolonged rainfall, causing water saturation of sediments and consequent failure of large sediment volumes (up to 800 000 m 3). Type 2 (2 debris flows) is triggered by short rainstorms which may destabilize the glacier drainage system, with debris flow volumes up to 100 000 m 3. Type 3 (6 debris flows) occurs during dry weather by glacial lake outbursts or ground/buried ice melting, with debris flow volumes up to 150 000 m 3. A data base of historic cases is needed in order to advance process understanding and modelling, and thus improve hazard assessment.